The present invention relates to a process for fusing a wide steel slab, in a longitudinal direction thereof, obtained by continuous casting or blooming so that surface abnormalities at widthwise edge portions of coils, particularly, of raw sheet to be surface-treated, which are produced by fusing the steel slab to obtain divided slabs, and further hot rolling and cold rolling the thus obtained slabs, may be prevented. (2) Related Art Statement:
In order to improve productivity in continuously casting plants, it is recently a common practice that a wide slab is cast in such a cast width that two or more of slabs having a required width can be obtained therefrom, the wide slab is cut with a torch at a given length, and lastly a plurality of slabs are produced by dividing the cut slab in a given width in a longitudinal direction by fusing it with gas or gas cutting
On the other hand, it is also a common practice that a so-called excess slab, which is produced by continuous casting or blooming without a specific order from customers, is cut in a given width over a longitudinal direction upon receipt of an specific order.
As shown in FIG. 8, since a surface portion at a fused plane 2 is partially melted and solidified again during gas fusion-cutting, a metal structure (solidified molten layer) 2a, which is different from a cast structure appearing on casting and solidification, is formed. Since this molten solidified layer is oxidized with oxygen gas or oxygen in air used for fusing during the fusing, a great amount of oxygen is dissolved into the melted surface portion. Consequently, as shown in FIG. 9, contents of alloying elements such as Al and Mn in the molten solidified layer become lower as compared with those of a slab base portion, so that physical, chemical and/or electrical properties of the former differ from those of the latter.
Consider that a cold rolled coil is obtained by hot rolling and cold rolling the above slab without removing the molten solidified layer, and then its surface is plated. In this case, as shown in FIG. 10, a white stripe (hereinafter referred to as "white band") 4, which has a luster different from that of the central portion, often appears over a width of dozens of millimeters from a widthwise edge of the coil 3, which edge corresponds to the divided face of the slab.
This white band is considered to be formed as follows:
When the above-mentioned molten solidified layer, formed produced at the fused surface portion, remains without being removed as a scale in hot rolling, the layer moves around opposite edge surface portions of the hot rolled coil owing to the flowing of the material at the edges during the hot rolling. The thus moved layer remains at the edge portion of the cold rolled coil in a band-like form. When the coil is plated, the band portion has electrochemical properties different from those of the surface layer of the coil central portion, that is, the surface layer of the slab base portion. Therefore, the state in which the plating metal is deposited on the coil, differs between the band portion and the central portion.
Since the above white band damages the external appearance of products, the band must be removed by cutting. Therefore, there are problems in that this greatly reduces yield of the products, and slows down the production line.
Further, when the above cold rolled coil is subjected to various treatment such as physical, chemical and/or electrical surface treatment even without being plated, a surface layer (white band) having different physical, chemical and/or electrical properties is formed at the edge surface portion of the cold rolled coil in the band-like form. Consequently, there is a great possibility that various surface abnormalities occur.